The invention relates to a method and apparatus for multiplexing signals. More particularly, the invention relates to a method and apparatus for de-multiplexing optical signals in the spatial and wavelength domains employing a dispersion device optically coupled to a random access two dimensional imager and employing software for sub-pixel interpolation.
Optical fiber sensor systems employ multiplexing techniques to allow the sharing of a source and processing electronics to reduce the per sensor cost and thereby improve the competitiveness of such systems. In addition, component sharing helps to reduce the overall weight of the system and enhances robustness. A variety of multiplexing technologies are known including spatial, wavelength, frequency and coherence domain multiplexing. However, the multiplexing capacity of any of these techniques is generally limited to about ten sensors due to various factors including speed, cross talk, signal to noise ratio and wavelength bandwidth. Some systems employ two or more techniques to increase multiplexing capacity. In particular spatial domain multiplexing is advantageously combined with other techniques, generally because it does not degrade system performance.
Fiber optic Bragg gratings (FBG) have become one of the most successful of the optical fiber sensors available. These devices are generally compact, have absolute wavelength encoding, and have the potential for mass production. Sensor signals may be wavelength encoded rather than intensity encoded. Thus the sensed signal is independent of power variations in the light source and system losses. Additionally, an array of FBG sensors can be readily made by connecting several FBGs having different center wavelengths in a line along a length of fiber. Each FBG may be individually addressed using wavelength multiplexing in the wavelength domain. However, wavelength domain alone can only accommodate a relatively small number of FBGs, because a broad band source optical fibers has only a limited bandwidth. Accordingly, it is desirable to employ combined multiplexing techniques to increase the capacity of the system.
Conventional spatial multiplexing locates sensors into many fiber channels and may employ a separate electronic signal processing unit for each channel. Such a system may be improved by using an optical fiber switch as a special case of spatial multiplexing, thereby allowing multiple fiber channels to share a single processing unit. However, the speed of the system, measured as the sample rate of each sensor is considerably reduced because of the optical switch, for example, 60 FBGs at a sample rate of 1 Hz. Certain applications such as monitoring aerospace structures or process control and massive data collection require higher multiplexing capacity, and particularly, a higher sampling rate are desirable.
A digital space and wavelength domain multiplexing technique, employing multiple fiber channels, sharing a processing unit, has been reported by the inventors herein. Single channel systems with multiple FBGs employ a dispersion device and line scan camera. The system is only a single domain device and the system is limited to one dimension and therefore it can only address a limited number of sensors.
The present invention seeks to overcome and obviate the disadvantages and limitations of the described prior arrangements. In particular, the invention is based upon the discovery that a large scale, high speed optical fiber sensor network may be provided which has wavelength and spatial multiplexing using a dispersion device an a two dimensional (2D) image sensor to distinguish a plurality of fiber channels on one axis and FBG wavelengths along another. An exemplary embodiment employs a random access 2D imaging device and a sub-pixel interpolation algorithm for resolution enhancement.
In an exemplary embodiment, the invention comprises the broad band source, a coupler for distributing the source to multiple fiber channels each including a plurality of fiber brag gratings (FBGs) therealong. The FBGs in each fiber each have a different center wavelength and the reflected signals from each FBG are carried by a down-lead fibers. The down-lead fibers are arranged along a line in a 1D array at the input port of a wavelength selective dispersion device. The reflected light is passed through the dispersion device which separates the reflected light by wavelength and directs the light to a 2D solid state image sensor. In the exemplary embodiment the 2D sensor is a random access device to thereby improve data acquisition speed. A sub-pixel interpolation algorithm is employed to enhance resolution.
In another embodiment the wavelength selective dispersion device, the 2D random access imaging device, and an in-line fiber optic input array may be combined as a module. A broad band source and a coupler for distribution to multiple fiber channels may be another module. The modules may be operated separately or combined in a single unit.
The FBG sensor is generally sensitive to both temperature and strain. In accordance with the invention. FBG arrays may be produced with temperature and strain sensitivity combined or separated. FBS array may also be adapted to sense physical characteristics which may be readily converted to a strain measurement.